Yost’s (2000) Fundamentals of Hearing provide a concise presentation of
the principles of acoustics for airborne sounds. Jensen et al.’s (1994) Computational Ocean Acoustics offers the most recent review of the general field
of underwater acoustics and is a wonderful companion to Urick’s (1983)
Principles of Underwater Sound. Lastly, Bradbury and Vehrencamp’s (1998)
Principles of Animal Communication offers an extensive biologically oriented discussion of acoustic signaling more specifically in the context of
animal communication, behavioral ecology, and the functional design of
sound-producing organs and receivers.
2. Sound
According to Speaks (1992, p. 2), “there is one principal prerequisite for a
body to be a source of sound—it must be able to vibrate. If a body is set
into vibratory motion, it must have the physical properties of mass and
elasticity and all bodies in nature possess both of these properties to
some degree. Hence, sound is a mechanical disturbance that displaces the
molecules of a medium whether the medium is a solid, liquid, or gas. A
vibration results in the distortion or acceleration of an object and is
established by the opposing forces of inertia (determined by an object’s
mass) and elasticity (an object’s ability to recover its original form following distortion). “Because all molecular structures have some finite mass
and elasticity, all are capable of being both a source of sound and a medium
for its transmission” (Speaks 1992, p. 2). Sound may therefore also be
defined either as “the propagation of density changes through an elastic
medium . . . [or] the transfer of energy through an elastic medium” (Speaks
1992, p. 41).
Within a sound field, one molecule collides with its neighbor, returns
back toward its initial position in the opposite direction, and collides with
another molecule. The cycle of collisions and returns through the initial
position is determined by the medium’s elasticity, and this oscillation can
be described by a sinusoid (Fig. 2.1). Each cycle of the sound wave defines
the periodicity of change in the amplitude of the sound’s vibration around
a baseline of ambient amplitude that may be characterized by any number
of quantities, including pressure, displacement, velocity, and acceleration. A
sound wave’s frequency (f) is the number of cycles/second, which is defined
in Hertz (Hz); its period (T) is the reciprocal of frequency, or 1/f, and is the
duration of a single cycle, measured in seconds. The distance covered by
one full cycle of the sound wave is its wavelength (l), which is equal to c/f
or cT, where c is the speed of sound propagation.
Most natural sounds are not pure sinusoids but complex, nonsinusoidal
waveforms. Complex waveforms can be represented by the sum of a
weighted series of sinusoids, i.e., the Fourier transform (Bradbury and
Vehrencamp 1998). Fourier analysis often reveals that a complex sound is
16
A.H. Bass and C.W. Clark
the principles of acoustics for airborne sounds. Jensen et al.’s (1994) Computational Ocean Acoustics offers the most recent review of the general field
of underwater acoustics and is a wonderful companion to Urick’s (1983)
Principles of Underwater Sound. Lastly, Bradbury and Vehrencamp’s (1998)
Principles of Animal Communication offers an extensive biologically oriented discussion of acoustic signaling more specifically in the context of
animal communication, behavioral ecology, and the functional design of
sound-producing organs and receivers.
2. Sound
According to Speaks (1992, p. 2), “there is one principal prerequisite for a
body to be a source of sound—it must be able to vibrate. If a body is set
into vibratory motion, it must have the physical properties of mass and
elasticity and all bodies in nature possess both of these properties to
some degree. Hence, sound is a mechanical disturbance that displaces the
molecules of a medium whether the medium is a solid, liquid, or gas. A
vibration results in the distortion or acceleration of an object and is
established by the opposing forces of inertia (determined by an object’s
mass) and elasticity (an object’s ability to recover its original form following distortion). “Because all molecular structures have some finite mass
and elasticity, all are capable of being both a source of sound and a medium
for its transmission” (Speaks 1992, p. 2). Sound may therefore also be
defined either as “the propagation of density changes through an elastic
medium . . . [or] the transfer of energy through an elastic medium” (Speaks
1992, p. 41).
Within a sound field, one molecule collides with its neighbor, returns
back toward its initial position in the opposite direction, and collides with
another molecule. The cycle of collisions and returns through the initial
position is determined by the medium’s elasticity, and this oscillation can
be described by a sinusoid (Fig. 2.1). Each cycle of the sound wave defines
the periodicity of change in the amplitude of the sound’s vibration around
a baseline of ambient amplitude that may be characterized by any number
of quantities, including pressure, displacement, velocity, and acceleration. A
sound wave’s frequency (f) is the number of cycles/second, which is defined
in Hertz (Hz); its period (T) is the reciprocal of frequency, or 1/f, and is the
duration of a single cycle, measured in seconds. The distance covered by
one full cycle of the sound wave is its wavelength (l), which is equal to c/f
or cT, where c is the speed of sound propagation.
Most natural sounds are not pure sinusoids but complex, nonsinusoidal
waveforms. Complex waveforms can be represented by the sum of a
weighted series of sinusoids, i.e., the Fourier transform (Bradbury and
Vehrencamp 1998). Fourier analysis often reveals that a complex sound is
16
A.H. Bass and C.W. Clark